1. Field of the Invention
The present invention relates to a light quantity adjuster that adjusts the quantity of light input via a lens, and an imaging apparatus using the light quantity adjuster.
2. Description of the Related Art
An imaging apparatus, such as a video camera or still camera, is provided with a light quantity adjuster that adjusts the quantity of light input via a lens. In general, the light quantity adjuster has a blade member disposed on the optical axis of the lens, and a drive section that moves the blade member to change the size of the aperture.
A stepping motor, for example, is used as the drive section of the light quantity adjuster.
An imaging apparatus according to a related art drives the stepping motor of the light quantity adjuster always at a constant speed to change the state of the aperture from an open state to a closed state, thereby capturing a still image. When the amount of movement of the blade member from the open state of the aperture to the closed state thereof is large, a change in the quantity of light passing the aperture forms a wavy curve, resulting in a longer exposure time.
When the exposure time (shutter speed) becomes longer, the reproducibility of exposure drops, degrading the quality of the image. When the shutter speed becomes greater than the period of a vertical sync signal of the imaging apparatus, changeover of images occurs during exposure, so that exposure control of the imaging apparatus becomes complex.
A change in the quantity of light passing the aperture will be described referring to FIG. 7. FIG. 7 is a diagram showing the relation between the actual change in light quantity when the stepping motor is driven always at a constant speed, and a change in light quantity instructed by a microprocessor.
For example, a frictional resistance occurs between the drive shaft of the stepping motor and a member that transforms the rotation of the drive shaft to the movement of the blade member. Accordingly, the actual change in light quantity starts with a delay from the timing (triggering) at which an instruction indicating the start of the operation is transmitted to the stepping motor (state a). Then, the drive speed of the stepping motor becomes faster to catch up with the instructed change in light quantity, so that the actual change in light quantity is faster than the instructed change in light quantity (state b).
When the actual change in light quantity catches up with the instructed change in light quantity thereafter, the actual change in light quantity cannot become greater than the instructed change in light quantity, so that the drive speed of the stepping motor becomes slower to cause the actual change in light quantity to become substantially equal to the instructed change in light quantity while making a transient response (state c). Because the actual change in light quantity transiently responds to the instructed change in light quantity, the actual change in light quantity becomes a wavy curve, making the exposure time longer.
One way to avoid the prolongation of the exposure time is to make the electric period of the stepping motor shorter. A technique of shortening the electric period of a stepping motor is described in, for example, JP-A-2004-12938 (Patent Document 1). According to the technique described in Patent Document 1, the conduction phase that is ahead of the phase of conduction to the coil in the light quantity adjustment state, which is determined by the condition of conduction to two sets of electromagnets, in the closing direction by 90 degrees is set as an initial conduction phase.